[web_dashboard] add logout (#447)

* logout wip

* Use AnimatedSwitcher, change lingo from "login" to signIn"

* add automatic sign-in

* fix flashing sign in button

* sign out of FirebaseAuth and GoogleSignIn

* formatting

* change isSignedIn() to getter

* Add error handling for sign in

* improve error handling at login screen

experimental/web_dashboard/lib/src/auth/auth.dart

@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 abstract class Auth {
+  Future<bool> get isSignedIn;
   Future<User> signIn();
   Future signOut();
 }
@@ -10,3 +11,5 @@ abstract class Auth {
 abstract class User {
   String get uid;
 }
+
+class SignInException implements Exception {}